Mating Type, Virulence, Aggressiveness and Metalaxyl Resistance of Isolates of Phytophthora Infestans in Poland

A total of 93 single-lesion isolates of Phytophthora infestans were collected from late blight diseased potatoes grown in several locations in Poland, in the years 2002–2004. They were tested for mating type, virulence, aggressiveness and metalaxyl resistance. Sixty one percent of the isolates was of the A1 and 39% of the A2 mating type. Regarding virulence, complex races predominated. Most of the isolates were highly and moderately aggressive. Among isolates, tested together with 38 previously collected (1995–2001) ones, 14.5% was metalaxyl resistant, 3.5% intermediately resistant and 82.0% of the isolates was sensitive to metalaxyl. In addition, 14 isolates tested with three simple sequence repeat markers showed high genetic diversity. These data indicate that Polish population of P. infestans is diverse and aggressive, with potential for sexual recombination and spreading of fungicide insensitivity.     

Identification of mating types and metalaxyl resistance in North American populations ofPhytophthora infestans

The A² mating type ofPhytophthora infestans was first reported in the United States in 1990. Concurrently,P. infestans strains resistant to metalaxyl ere found in the Pacific Northwest. Collaborative surveys were undertaken during 1991–1993 to investigate the frequency of occurrence of A² mating types and metalaxyl resistant strains in populations ofP. infestans isolated from outbreaks of late blight in potato and tomato crops in North America.In vitro testing indicated that isolates from the northeastern U.S. and Atlantic Canada were primarily (52/55) metalaxyl sensitive and all were A¹ mating types. Among 85 isolates from late blight epidemics in Florida and Texas, greater than 61% were both metalaxyl resistant and A² mating type. Metalaxyl resistance and A² mating types were identified also in a few tomato isolates from North Carolina. Although the majority of 134 isolates from the Pacific Northwest (British Columbia and Washington) were metalaxyl resistant, only 2 isolates from Washington were A² mating types. Among 111 isolates from 2 sites in central Mexico, 63% and 77% were both metalaxyl resistant and A² mating types. The data indicate also a higher frequency of metalaxyl resistance in A² isolates, than in A¹ isolates, among isolates from Florida and Texas. Highest metalaxyl resistance levels were found, however, in A¹ isolates from California, where no A² isolates were recovered.     

Characterisation of Phytophthora infestans Isolates Collected from Potato and Tomato Crops in Tunisia During 2006–2008

Severe late blight epidemics in Tunisia in recent years prompted population studies on the pathogen responsible for this disease, Phythophthora infestans. Characterisation of 165 Tunisian P. infestans isolates collected from 2006 to 2008 was performed for the mating type and mt haplotype, while subsets were analysed for metalaxyl sensitivity (n?=?65), virulence on differential set of 11 R genes of Solanum demissum (n?=?31), aggressiveness on cv. Bintje (n?=?36) and measurement of the radial growth on agar medium at three temperatures (n?=?38). Most isolates from potato and all isolates from tomato had the A1 mating type. The A2 mating type was detected in the north-east and northern areas, but not in the north-west. All the A2 mating type isolates were metalaxyl resistant and seem to be part of a new generation of the P. infestans isolates which are more aggressive, with more complex races, and tolerant to higher temperatures. The increased severity of epidemics during 2006 to 2008 can be attributed to favourable weather conditions during growing seasons, adaptation of new genotypes, widespread phenylamide resistance in potato production regions and most probably incorrect spray programmes. In contrast to the presence of complex pathotypes in two major potato crop regions (north-east and northern areas), the P. infestans population detected in the other regions and in tomato crops was still relatively simple. Compared with the situation in Europe and the American continent, or even compared with neighbouring countries such as Algeria, the genetic changes in Tunisia are still comforting and require strict management decision on late blight control to avoid the spread of new P. infestans populations from Europe or neighbouring countries.     

The Pathosystem Solanum tuberosum L.-Phytophthora infestans (Mont.) de Bary in Chapingo,Mexico. Expected,Observed, and Simulated

The 1:1 ratio of the Phytophthora infestans A1 and A2 mating types favors the sexually derived genetic diversity of the late blight pathogen of potatoes, which is widespread in the central highlands of México. This ratio guarantees the successful infection of this pathogen, even in resistant potato hosts. However, wild Solanum species present in the region serve as alternative hosts to the pathogen. Knowledge about the external factors that influence the dynamics of this disease facilitates the assessment and selection for genetically resistant potato cultivars to late blight, in addition to providing the capacity to predict epidemics. This work aimed to assess the expected, observed, and simulated progress of natural P. infestans infection of potatoes during two epiphytotic field seasons (2009 and 2010) at Chapingo, México. Using 8-years of weather datasets, six ideal situations were predicted with four to eight infection cycles of 6 to 12 h each. In comparison to the predictions, the observed effect of the area under the disease progress curve, and its components (AUDPC, RAUDPC, RaRAUDPC), was highly significant, with a low coefficient of variation among the potato cultivars used in the study. In conclusion, we confirm that the LATEBLIGHT-LB2004 model is useful for simulating and predicting late blight epidemics based on the weather conditions of Chapingo, except for the magnitude of the relative humidity threshold variable (RH_threshold), which requires calibration for each cultivar.     

Assessment of mating types and resistance to metalaxyl of Canadian populations ofPhytophthora infestans in 1997

The distribution of A1 and A2 mating types, and of metalaxyl-sensitive (MS), metalaxyl-intermediate (MI), and metalaxyl-resistant (MR) strains ofPhytophthora infestans, the causal pathogen of late blight, were determined in Canada in 1997. Potato plants or tubers and tomato plants or fruits, suspected to have late blight infection, were received from seven Canadian provinces in 1997, and were assessed for the presence ofP. infestans. Among 220 samples suspected to have late blight,P. infestans was recovered from 132 samples (60%) received from British Columbia (BC), Manitoba, New Brunswick, Prince Edward Island, and Quebec. From the 132 samples, 364 isolates were obtained and tested for their mating type and response to metalaxyl. Populations ofP. infestans within Canadian provinces were homogeneous for mating types, except within BC, where populations comprised both A1 and A2 mating types both within fields and sometimes within samples. The A2 mating type was found in all provinces where late blight was present. The A1 mating type was found only in Manitoba (1 isolate) and in BC (38 isolates). Unlike the old A1 isolates, those found in BC in 1997 were highly resistant to metalaxyl. These A1 isolates are even more resistant than A2 isolates. Based on assays with 10 ug.ml^?1 of metalaxyl, 14% of all tested isolates were MS, 69.8% were MI and 16.2% were MR. In addition, all provinces contained MS, MI, and MR isolates. No specific mating type or metalaxyl sensitivity level were specific to tomato or potato.     

Relationships between pathotypes and RAPDs, Gpi-allozyme patterns, mating types, and resistance to metalaxyl ofPhytophthora infestans in Canada in 1997

Relationships between pathotypes and other markers such as mating types, metalaxyl resistance, Gpi-allozyme and random amplified polymorphic DNA (RAPD) patterns, were investigated among 33 isolates ofPhytophthora infestans obtained from both potato and tomato across Canada in 1997. Pathogenicity testings, based on virulence against potato differentials carrying single-resistance genes and the size of lesions on leaves, revealed higher pathogenic diversity in 1997 than in the few past years, with more pathotypes and more race-complex isolates. A higher pathogenic diversity was observed within A2 (US-8) than within A1 (US-11) isolates. There was no correlation between the level of resistance to metalaxyl of an isolate and its pathogenicity. Nevertheless, when grouped into metalaxyl-sensitive (MS), -intermediate (MI), and -resistant (MR) classes, both average pathotype complexity and average lesion size were lowest for MS group. RAPD data revealed a good distinction between A1 (US-11) and A2 (US-8) mating type groups, and between isolates from British Columbia and those from other provinces. The detection in the last few years of both A1 and A2 mating types in Canada and the occurrence in 1997 of some isolates sharing different characteristics of both of these two groups suggest that sexual reproduction may have occurred locally.     

Phenotypes ofPhytophthora infestans in Nepal: mating types and metalaxyl sensitivity

Summary Potato late blight has appeared in epidemic proportions in Nepal since the mid 1990s and fungicides have been reported to be decreasingly effective in managing the disease.Phytophthora infestans isolates were collected from potato crops during 1999–2000 and analysis of 371 isolates for mating types and 270 isolates for metalaxyl sensitivity revealed the presence of both new and old populations. This is the first report on the presence of metalaxyl-resistant isolates in Nepal. The frequencies of A1 and A2 were 83 and 17%, respectively. Metalaxyl-resistant, intermediate and sensitive isolates were recorded as 10%, 12% and 78% respectively. Metalaxyl resistance was distributed in both mating types. Sites with a history of metalaxyl use had a significantly higher number of resistant and intermediate isolates ({ie337-1}, P<0.01) than sites where metalaxyl had not been used. This study confirms the changes in population structure ofP. infestans in Nepal.     

Survey ofPhytophthora infestans populations in Serbia: Mating type, metalaxyl resistance and virulence properties

Summary A total of 37 single-lesion isolates ofPhytophthora infestans were collected during 2000 from different potato growing regions in the central part of Serbia. They were paired in Al and A2 matings to determine their mating types. Of these 37 isolates, 28 were found to belong to the A2 mating type, eight to the Al type and one was found to be self-fertile (A1/A2). Using the in vitro mycelial radial growth method no metalaxyl resistantP. infestans isolate could be found. The rate of occurrence of intermediate isolates was not high, on the average 2.70%. All other isolates were sensitive to metalaxyl. There was no correlation between mating type and the intermediate metalaxyl resistance found.     

Emerging and Re-emerging Potato Diseases in the Andes

Outbreaks of a new mating type (A2) of late blight (Phythopthora infestans) and the spread of bacterial wilt (Ralstonia solanacearum) were the most notable events among fungi and bacteria during the last 10 to 15 years in the Andes. The emergence of the A2 mating type of P. infestans was probably a result of seed movement from the Northern hemisphere. Black scurf caused by the fungus Rhizoctonia solani spread in Peru and Bolivia causing economic losses estimated at 5 to 10%. A high incidence of common scab on potato tubers, caused by the bacteria Streptomyces scabies was found in some localities in Bolivia. Among the emerging viruses, the NTN strains of PVY have been found in some countries including Peru. Recently, two other viruses have been found spreading in potatoes. One is coded SB26/29 and is transmitted by Russelliana solanicola, the first psyllid vector found for a plant virus. The other emerging virus is coded SB41 and is apparently also new to potatoes. It causes a severe mosaic on the affected plants and the virus is transmitted experimentally only by grafting. Some other viruses of minor importance have also been detected in potatoes.     

Potato fungicides interfere with entomopathogenic fungi impacting population dynamics of green peach aphid

Fungicides applied to potato can enhance green peach aphid,Myzus persicae (Sulzer), outbreaks by interference with entomopathogenic fungi. (Order Entomophthorales). Late season aphid numbers were highest in potatoes sprayed with metalaxyl + mancozeb, captafol, or mancozeb, and lowest in potatoes sprayed with benomyl, triphenyltin hydroxide, chlorothalonil, or copper hydroxide. In field-collected aphids,Pandora (= Ernyia) neoaphidis (Remaudière et Hennebert) andEntomophthora planchoniana Cornu (F. Entomophthtoraceae) were the predominant cause of mycoses, 66.7% and 22.3%, respectively.Conidiobolus obscurus (Hall and Dunn) Remaudièe and Keller (F. Ancylistaceae) accounted for 8.5% of mycoses. In the laboratory, fungicides were shown to have direct effects on these entomopathogens. Metalaxyl + mancozeb, mancozeb and captafol were strongly inhibitory of germination of conidia, copper hydroxide was intermediate, and chlorothalonil had little effect. Triphenyltin hydroxide, benomyl, metalaxyl + mancozeb, and mancozeb were strongly inhibitory of growth of mycelia, copper hydroxide was intermediate, and chlorothalonil and copper hydroxide had least effect. Benomyl was highly toxic to green peach aphid, copper hydroxide and chlorothalonil intermediate, and captafol, mancozeb, and metalaxyl + mancozeb least toxic. Possible interference of potato fungicides with aphid pathogens is now an important consideration because of the intensity of spraying required to protect the crop from infection by metalaxyl-resistant strains of the late blight pathogen,Phytophthora infestans (Mont.) de Bary. Minnesota potato growers reported high green peach pressure in both 1995 and 1996, years of intensive fungicide spraying. Concomitantly, there was a marked increased in the incidence of PLRV in seed lots entered for winter testing.     

Testing for resistance to metalaxyl inPhytophthora infestans isolates from northwestern Washington

Outbreaks of late blight occurred in commercial potato fields in northwestern Washington in 1989 and reached epidemic proportions in 1990. Since most blighted fields reportedly had received 2–4 late-season applications of metalaxyl, the possibility existed that pathogenic strains ofPhytophthora infestans had developed resistance to this systemic fungicide. In testing this hypothesis,P. infestans was isolated consistently from diseased leaves, stems, and tubers of potato when small tissue pieces were surface-disinfected in 0.5% NaOCl for 3 min. Virulence of isolates was assessed on detached leaflets of eight differential (R-gene) potato genotypes. Although simple races (single and double) were recovered, there was also a high frequency of complex race combinations. When tested in metalaxyl-amended media, 81% of the 73 isolates recovered from diseased tissues were highly resistant to 10 μg metalaxyl/ml and 19% yielded intermediate resistance. Sensitive isolates were not recovered. Fifty-five percent of 40 isolates sporulated on potato tuber disks in the presence of ≥10 ug metalaxyl/ml. On leaf tissues, 85% of 20 isolates sporulated in the presence of 10 μg metalaxyl/ml, and 50% sporulated in the presence of 100 μg/ml. This is the first report ofP. infestans resistance to metalaxyl in the U.S.     

Changes in metalaxyl resistance among glucose phosphate isomerase genotypes ofPhytophthora infestans in Canada during 1997 and 1998

A total of 930 isolates ofPhytophthora infestans collected during 1997 and 1998 from commercial potato and tomato fields, home gardens, and potato storages in Canada was analyzed for glucose phosphate-isomerase (Gpi) genotype, mating type, and resistance to metalaxyl. Both mating types and diversity in resistance to metalaxyl were detected within and amongGpi-genotypes. Pathogen populations were dominated by the 100:111:122Gpi-genotype (characteristic of US-8) in all the provinces where late blight was detected, except in 1997 in British Columbia (BC) where the 100:100:111Gpi-genotype (characteristic of US-11) predominated.Gpi-genotypes 100:111, 111:122, and 100:122 (characteristic of US-7, US-10 and US-14/US-17, respectively) were less common. Nearly all of the isolates with the 100:100:111Gpi-genotype were the A1 mating type, except for a few collected in BC in 1997. A majority of isolates with the 100:111:122Gpi-genotype were A2 except for a few found in Québec in 1998. Based on colony growth on media amended with 1,10, or 100 μg/ml metalaxyl, the 100:100:111Gpi-genotype isolates had higher levels of resistance to the fungicide in 1998 compared to 1997. In 1998, level of metalaxyl resistance was higher within the 100:100:111Gpi-genotype whereas it was lower within the 100:111: 122Gpi-genotype, when compared to 1997. BC was the only Canadian province where an increase in metalaxyl resistance within the 100:111:122Gpi-genotype was recorded between 1997 and 1998. Isolates from leaves had higher metalaxyl resistance levels than stem or tuber isolates, regardless of genotype.     

Controlling potato late blight with systemic-protectant fungicide combinations of metalaxyl and mancozeb

Disease development was monitored weekly in field plots inoculated withPhytophthora infestans and treated with metalaxyl, mancozeb or metalaxyl/ mancozeb combinations. Late blight pressure was moderate to severe in 1981 and 1983 and was slight to moderate in 1982 and 1984. In 1982–84 all treated plots had significantly less foliar damage and late blight tuber rot than untreated plots. Use of metalaxyl alone resulted in the greatest increase in disease (up to 80% defoliation) in the latter part of the growing season. Under slight to moderate disease pressure, metalaxyl/mancozeb and mancozeb treatments provided significantly better disease control than metalaxyl. Under moderate to severe disease pressure, high levels of disease (65% defoliation) occurred by the end of the season with mancozeb and metalzxyl/mancozeb on 7- and 14-day spray schedules, respectively. However, significantly better control (about 20% defoliation) was obtained during early season, rapidly developing epidemic conditions when only three applications of metalaxyl/mancozeb were included in a 7-day mancozeb spray program.     

Novel genotypes ofPhytophthora infestans in Canada during 1994 and 1995

Populations ofPhytophthora infestans, causing late blight of potato, changed significantly in Canada in the mid-1990s. The US-1 genotype (A1, metalaxylsensitive [MS]) was displaced by novel genotypes (mainly US-8) in much of the country in 1994 and 1995. Many of the new genotypes were insensitive to metalaxyl and were of the A2 mating type, although metalaxyl-insensitive (MI) Al isolates of the gll (or US-11) genotype were found in British Columbia. A total of 14 unique multilocus genotypes were described over the two years of the study based on mating type (Al or A2), metalaxyl sensitivity (MS or MI), allozyme banding patterns (Gpi andPep), and random amplified polymorphic DNA (RAPD) markers. Six of these genotypes could be distinguished by allozyme banding patterns at theGpi locus alone; RAPD analysis revealed additional variation within allozyme groupings and defined additional genotypes. Among 82 isolates collected from various Canadian provinces in 1994, US-1 and US-8 constituted 30% and 49% of isolates, respectively; other genotypes occurred in minor frequency. Among 50 isolates collected in 1995, US-1 and US-8 constituted 10% and 74%, respectively. Isolates of gll (US-11) were found in British Columbia in 1995. Diversity of genotypes was greater in 1994 (11 genotypes) than in 1995 (six genotypes). We hypothesize that sexual reproduction may have been responsible for generating some of the genetic variability inP. infestans populations, during a time period in which A2 isolates first migrated into many provinces in Canada.     

Multiplication and rate of reinfection of virus-free potatoes in Prince Edward Island

When five varieties of virus-free potatoes were introduced and multiplied for three years at an elite seed farm in Prince Edward Island where virus-infected potatoes were also grown, approximatly 10% of each variety became infected with both potato virs S (PVS) and potato virus X (PVX). However, when eight varieties of similar stocks were multiplied for 2 years at private farms in the absence of virus-infected potatoes, all remained free from PVX. Four of the varieties also remained free from PVS and four became infected with amounts of less than 5%. In other tests on these stocks, no potato virus Y (PVY) was detected. Post harvest tuber testing onNicotiana debneyi Domin. was found to be more reliable in detecting recent infections of PVS than foliage tests done during the growing season. This was so whether the foliage tests were done on the indicator plant, serologically or doubly-tested by both methods.     

Virulence Complexity and High Levels of Fungicide Resistance Suggest Population Change of Phytophthora infestans in the Heilongjiang Province of China

Samples of Phytophthora infestans were isolated from 12 city and county fields in the main potato production areas of Heilongjiang province of China between 2004 and 2008. A total of 72 isolates were tested for virulence on a potato R gene differential set and 39 of these isolates were also analysed for their resistance to the fungicide metalaxyl. All 11 differential plants were infected by one or more of the isolates collected; the most commonly overcome differentials were R1, R3, R4, R7, R10, and R11. Most (89.7%) of the tested isolates were resistant to metalaxyl and the rest (10.3%) were sensitive. The proportion of metalaxyl-resistant isolates differed between sites (χ ²?=?25.42, df?=?10, p?<?0.005), but not between years (χ ²?=?4.63, df?=?4, p?>?0.05). Taken together and in comparison with earlier pathogen population studies, our findings suggest that the population of P. infestans in Heilongjiang is different from that existing in the province two decades ago and that at various locations within the province, the new population has a high frequency of resistance to metalaxyl.     

Thermal properties of overwintered piles of cull potatoes

Annual epidemics of late blight of potato have lead to debate as to the relative importance of overwintering sources of inoculum. Host availability is a key factor for overwinter survival ofPhytophthora infestans and the initiation of epidemics of potato late blight. Temperature within discarded piles of cull potatoes may influence tuber tissue temperature and therefore affect survival of meristematic tissue. Consequently the risk of initiation of an epidemic of late blight from cull piles is closely related to the thermal experience of overwintered potato culls. Temperature monitoring of cull piles over two years indicated that temperature was stable in the interior of the piles regardless of cull pile size (1–15 ton) or year. The possible use of ambient temperature information gathered during winter months in potato-growing regions may not help estimate the risk of an epidemic of late blight initiated from cull piles as, although highly correlated with the surface temperature of cull piles, ambient temperature was not correlated with the temperature within the cull piles. The presence of cull piles in excess of 1 ton may enhance the survival of the host and thus the development ofP. infestans from infected tubers.     

Race diversity and virulence complexity ofPhytophthora infestans in Nepal

Summary A total of 251Phytophthora infestans isolates collected during 1999–2000 potato growing seasons from nine potato growing areas of Nepal were tested for virulence using a set of eleven potato differentials. The virulence test revealed the presence of 30 races with the most frequent being 4.11, 4 and 11. Twenty-nine percent of isolates belonged to simple races having none to one virulence factor, whereas 71% had complex virulence factors governed by 2 to 7 genes. Gleason and Shannon indices of 5.24 and 2.59, respectively, showed a moderate level of diversity in NepaleseP. infestans populations. Shannon indices comparison among regional populations revealed significant differences (t-test, P<0.001) in the diversity. The population from hills were the most diverse followed byterai and the high hill populations. The virulence complexities were minimal in Nepalese populations. The implications of these findings in association with the potato cultivation system in Nepal are discussed.     

Performance of Dimethomorph + Mancozeb Applied to Seed Potatoes in Early Management of Late Blight (<Emphasis Type="Italic">Phytophthora infestans</Emphasis>)

Treating seed potatoes with contact fungicides prevents infection of Phytophthora infestans from tuber to tuber during handling but does not prevent field infections in the young plant. Dimethomorph (DMM) 9% + mancozeb 60% (Acrobat MZ, BASF) can cover both aspects due to the contact and systemic action of mancozeb and DMM, respectively. Experiments were performed under greenhouse and field conditions (three seasons and two locations) with cvs Kennebec, Shepody, Spunta, Russet Burbank, and Ranger Russet. Immediately after cutting, seed pieces were treated with Acrobat MZ at doses equivalent to 2, 3, and 4 kg ha⁻¹. At any dose, Acrobat MZ protected healthy seed tubers against P. infestans infections. Acrobat MZ (4 kg ha⁻¹) also protected the foliage of all cultivars against artificial inoculation of P. infestans, assessed by the detached-leaf method. These effects were observed up to 30 days after emergence in greenhouse experiments and up to 28 days after crop emergence under field conditions in two growing seasons. In cv Spunta, significant foliage protection was observed up to 35 days after field emergence whereas in seed tubers of cv Kennebec planted in sandy soils, significant foliage protection persisted longer. Additional evidence for acropetal translocation of Acrobat MZ in field crops was also found. With this strategy, there is no need to apply contact or systemic fungicides early in the season, and spraying costs and environmental risks are also reduced.     

Occurrence of the A2 mating type ofPhytophthora infestans in potato fields in the United States and Canada

Ten isolates ofP. infestans collected from blighted potato fields in the U.S. and Canada during the years 1983–1989 were examined for mating type on lima bean, oatmeal, and rye grain media. Two of the ten isolates produced oospores in less than 15 days when cultured in the presence of known A¹ mating types from the USA, Mexico, and Europe, indicating they are A² mating types. When the two A² isolates (one from Pennsylvania and one from Vancouver, B.C.) were cultured singly or with known A² isolates, no oospores formed after 6 weeks of culture. Pathogenicity tests showed that both isolates produced typical late blight symptoms on potato foliage and stems, and there appeared to be no difference in virulence between these two A² and the eight A¹ isolates. Inoculations with a mixture of A¹ + A² sporangia also produced oospores in host tissues. This is the first report of the presence of the A² mating type ofP. infestans in the U.S. and Canada.